A single wave washer 1a shown in FIGS. 6A and 6B is used for a bearing device of a general electric motor as described in JP-A-9-287620, JP-A-8-103049 and JP-A-2000-87964, for example. In the general electric motor, an axial load due to vibration during rotation is small. However, a strong vibration is applied to a vehicle AC generator due to variation of a vehicle engine and variation due to a high speed rotation of the AC generator itself. Furthermore, when the AC generator has a large electrical generating capacity, the weight of its rotor increases and a creep force applied to a bearing of a rotation shaft of the AC generator also increases. In this case, a large creep resistance force is necessary for this AC generator.
If the single wave washer 1a is used for the vehicle AC generator, in order to have a large creep resistance, a pre-load of the single wave washer will exceed an allowable pre-load limit. As a result, the vehicle AC generator may be easily damaged. Therefore, it is difficult to satisfy both the large creep resistance and the allowable pre-load limit when a single wave washer is used.